Added helper function for inline error messages

Added helper function for inline error messages

The previous code fragments were mostly out-of-sync and were slightly
differing from each other. The new helper function `elInnerError()`
provides a consistent and predictable behavior:

elInnerError(el, 'Hello World') -> Shows the inline error message "Hello
World" right after the element `el`.

Removing the inline error can be achieved through one of these calls:
elInnerError(el, '')
elInnerError(el, false)

For the sake of code simplicity, these two calls will also the inline
error, but it is strongly recommended to use the above constructs to
keep the intention clear.
elInnerError(el, null)
elInnerError(el)